In 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), a Zadoff-Chu sequence (hereinafter “ZC sequence”) is adopted as a reference signal (hereinafter “RS”) used in uplink. This ZC sequence is a kind of CAZAC sequence (Constant Amplitude and Zero Auto-correlation Code) and represented by following equation 1 or 2.
                    [        1        ]                                                                                  a            r                    ⁡                      (            k            )                          =                  {                                                                                          ⅇ                                                                  -                                                                              j                            ⁢                                                                                                                  ⁢                            2                            ⁢                            π                            ⁢                                                                                                                  ⁢                            r                                                    N                                                                    ⁢                                              (                                                                                                            k                              2                                                        /                            2                                                    +                          qk                                                )                                                                              ,                                                                              N                  ⁢                                      :                                    ⁢                  even                                                                                                                          ⅇ                                                                  -                        j                                            ⁢                                                                                          ⁢                                                                        2                          ⁢                          π                          ⁢                                                                                                          ⁢                          r                                                N                                            ⁢                                              (                                                                                                            k                              ⁡                                                              (                                                                  k                                  +                                  1                                                                )                                                                                      /                            2                                                    +                          qk                                                )                                                                              ,                                                                              N                  ⁢                                      :                                    ⁢                  odd                                                                                        (                  Equation          ⁢                                          ⁢          1                )                                [        2        ]                                                                                  a            r                    ⁡                      (            k            )                          =                  {                                                                                          ⅇ                                          j                      ⁢                                                                                          ⁢                                                                        2                          ⁢                          π                          ⁢                                                                                                          ⁢                          r                                                N                                            ⁢                                              (                                                                                                            k                              2                                                        /                            2                                                    +                          qk                                                )                                                                              ,                                                                              N                  ⁢                                      :                                    ⁢                  even                                                                                                                          ⅇ                                          j                      ⁢                                                                                          ⁢                                                                        2                          ⁢                          π                          ⁢                                                                                                          ⁢                          r                                                N                                            ⁢                                              (                                                                                                            k                              ⁡                                                              (                                                                  k                                  +                                  1                                                                )                                                                                      /                            2                                                    +                          qk                                                )                                                                              ,                                                                              N                  ⁢                                      :                                    ⁢                  odd                                                                                        (                  Equation          ⁢                                          ⁢          2                )            Here, N is the sequence length and r is the ZC sequence number, and N and r are coprime integers. Further, q is an arbitrary integer. Reasons to adopt a ZC sequence as an RS include constant frequency response characteristics, good auto-correlation characteristics and low PAPR (Peak to Average Power Ratio).
Further, if the sequence length N of a ZC sequence is a prime number, N−1, a number proportional to N, ZC sequences of good cross-correlation values can be generated. At this time, the cross-correlation value with respect to the signal levels between sequences of the same sequence length (e.g. between different ZC sequence numbers r=1 and r=5) is 1/√N, and the cross-correlation value decreases when the sequence length N is longer.
Meanwhile, amongst RSs used in uplink, transmitting a reference signal for channel estimation used to demodulate data (hereinafter “DM-RS,” which stands for demodulation reference signal) in the same band as the data transmission bandwidth, is adopted in 3GPP LTE. For example, if the data transmission bandwidth is one RB (resource block), the transmission bandwidth of a DM-RS is also one RB, and, if the data transmission bandwidth is two RBs, the transmission bandwidth of a DM-RS is also two RBs. By defining the sequence length N in advance, the transmission bandwidth (the number of RBs) and the sequence length are associated uniquely. For example, N is defined as a prime number to be less than and closest to the number of subcarriers forming an RB. In this case, when one RB is formed with twelve subcarriers, a DM-RS using one RB uses a ZC sequence with a length of which sequence length N is 11, and a DM-RS using two RBs uses a ZC sequence with a length of which sequence length N is 23. In this way, the transmission bandwidth (the number of RBs) and the sequence length are associated uniquely, and the sequence length N of a ZC sequence is longer when the transmission bandwidth (the number of RBs) is wider.
Here, the data transmission bandwidth is determined based on the scheduling of each cell, DM-RSs of different transmission bandwidths are transmitted in the same band between the cells. In this way, when ZC sequences of different transmission bandwidths (different sequence lengths) are multiplexed in the same band, the cross-correlation increases significantly in a certain specific combination of sequence numbers. FIG. 1 shows cross-correlation characteristics obtained by computer simulations. The X axis shows delay (symbols) and the Y axis shows auto-correlation values normalized by signal levels, and the results show the correlations of ZC sequence of N=23 and r=1 to 6 with respect to ZC sequence of N=11 and r=3. As shown in FIG. 1, the correlation value in the combination of N=11 and r=3, and N=23 and r=6 is 0.9 at the maximum, and shows near the signal level, that is, 1.0. The cross-correlation increases about three times as much as the cross-correlation value in the same transmission bandwidth, that is, 1/√N.
As shown in FIG. 2, if a combination of ZC sequences that increases a cross-correlation (e.g. above-described (r=3 and N=11) and (r=6 and N23)) is allocated to a nearby cell, the influence of interference of a DM-RS from the other cell increases and the accuracy of channel estimation significantly deteriorates, and therefore, demodulation performance deteriorates.
Then, Non-patent Document 1 discloses a ZC sequence hopping method in cellular radio communication systems. Non-patent Document 1 suggests randomizing (i.e. making uniform and equalizing) the interference mobile stations receive from other cells by making sequence numbers of ZC sequences used in DM-RSs a predetermined hopping pattern, and preventing deterioration of modulation performance because of receiving persistently significant interference in one mobile station from other cells.
FIG. 3 shows the hopping example disclosed in Non-Patent Document 1. First, ZC sequences are allocated on a per transmission bandwidth basis (on a per number of RBs basis or sequence length basis) in a predetermined rule, and the allocated ZC sequences are allocated as one sequence group to a cell. Then, by switching the sequence groups at predetermined switching time intervals and a predetermined hopping cycle, interference from other cells is randomized.
FIG. 3 shows that a sequence group in which ZC sequences of certain sequence numbers is formed and that the sequence group is switched at one-slot time intervals and in an M-slots hopping cycle. By this means, numbers of ZC sequences used in each cell are switched with time, it is possible to prevent a certain mobile station from receiving significant interference from other cells persistently and randomize the influence of interference from other cells.
Non-Patent Document 1: Huawei, R1-071109, “Sequence Allocation Method for E-UTRA Uplink Reference Signal,” 3GPP TSG RAN WG1Meeting #48, St. Louis, USA, Feb. 12-16, 2007